As the technology in the semiconductor field develops there are demanded semiconductor single crystals which are manufactured according to the FZ process, particularly silicon single crystals, having larger diameter, and at present even such single crystals as having 6 inch diameter are manufactured. The diameter of polycrystal silicon to be a rod for obtaining single crystals of such a large diameter is not necessarily the same as that of a single crystal to be obtained because, for example, in the case of using a slender rod, it is possible to enlarge the diameter of the rod due to the technique with a stuffing operation. However, the smaller in diameter the more the longer rod is required, and a problem of charging the longer rod into the FZ machine arises, so that a blank having a diameter almost same as that of the single crystal intended to obtain is required.
As are disclosed in Japanese Patent Publication Nos. 12358/78 and 23457/80, the polycrystal silicon to be a rod for manufacturing a silicon single crystal is obtained in such a way that for example, a monosilane gas is supplied onto electrically heated filament cores and then thermally decomposed for deposition thereon. If it is intended to obtain polycrystal silicon which is crushed for use, it is most general for a bridge for electrically connecting filament cores in the manufacturing equipments to use silicon materials described in said publication by using the carbon referred to in Publication No. 23457/80 even from the viewpoints of preventing the bridge from contamination by heterogeneous material and of subsequent processing.
In such a case of using silicon materials, polycrystal silicon deposits on the filament cores and the connection bridge to the same amount as the polycrystal silicon. However, as shown in FIG. 4 (II), as the deposition develops and the connection bridge portion thickens, the gas flow within the thermal decomposition furnace varies, the polycrystal silicon which deposits on the filament cores is deformed particularly in the neighbourhood of the joints of the connection bridge with the filament cores, when said joints become slender than other portions and at the same time the initial central axis is moved. A polycrystal silicon rod used in the FZ process needs to be of uniform diameter, so that when such a rod is used in the FZ process deposition must be continued until the slender portions increase their diameter to satisfaction, and then the outer diameter of the rod must be ground so as to obtain a predetermined diameter. Naturally, the larger the diameter, the more the grinding loss increases even in the same grinding ratio, and the more the diameter into a rod changes, the more the grinding loss increases.
Referring to the grinding of the outer diameter, it does not answer the purpose that the outer periphery of the rod becomes uniformly thick, but the rod is ground up to a predetermined diameter so that the resistivity of the intended single crystal may become a predetermined value. Such grinding is employed in any case of embedding or coating of impurities to the outer surface of a polycrystal rod, doping by gaseous impurities, and control between the resistivity of the filament cores and the resistivity to a finished product whose diameter has increased to as desired.
For these reasons it is specifically necessary for polycrystal silicon used in the FZ process that the crystals are of uniform diameter from their manufacturing state in order to realize shortening of growth time, reduction of material or electric charges, reduction of processing steps, reduction of grinding loss, and the like.
However, sufficient measures for solving such problems have not so far been taken.